I can't give you an exact answer, but my flights with Thai the other week ONLY asked for my passport. They didn't even look at my printed itinerary with barcode that I placed on the counter. AirAsia at DMK didn't even ask for credit card or look at the printed itinerary. They only wanted my passport.

These were all international flights and occurred within the last 60 days and just my personal experience. I do like how you have the letter from the card issuer. True initiative ^

What if your credit card that you used to purchase those tickets was stolen the day before your departure? Are they going to deny you boarding because your credit card was stolen? Personally, I doubt it.